It is proposed to further investigate the mechanisms of adaptation in the ventral photoreceptors of the horseshoe crab, Limulus. The hypothesis that a rise in the intracellular free calcium concentration, (Ca ions)-in, mediates light adaptation has been supported by a variety of experiments. When, however, (Ca ions)-in was measured by a fairly direct technique, it was found that changes in (Ca ions)-in and sensitivity were not correlated. As a resolution to this problem, a refinement of the calcium hypothesis is proposed in which the rise in (Ca ions)-in causes an increase in the occupation (by calcium) of sites which regulates sensitivity. Given certain physical assumptions about the kinetics of calcium binding to these sites, it is possible that the statistical occupation of the sites, (by calcium) correlates with sensitivity, even though (Ca ions)-in does not. To test this hypothesis, it would be desirable to have a measure of site occupation. It is argued that if the sites are on the inner surface of the membrane, the binding of calcium ions to the sites must alter the surface potential. Although these potentials cannot be measured directly by microelectrodes, there are techniques available for measuring surface potentials. It is proposed to use such techniques in order to further test the calcium hypothesis.